1. Field of the Invention
The present invention relates to a magnesium alloy cast material for plastic processing, a magnesium alloy member using the same, and a manufacturing method thereof.
2. Description of Related Art
A magnesium alloy has a specific gravity of about 1.8, which is 2/3 times as heavy as an aluminum alloy, and is highly expected as a substitute for reducing the weight of members. At the present, however, parts are mainly manufactured by die casting, and applicable parts are mostly parts of low strength such as cases and covers. If, therefore, materials applicable to high strength parts and method of manufacturing at low cost are established, the industrial value will be significant.
Existing magnesium alloys are generally poor in forgeability, and ZK60 alloy relatively excellent in formability is used in forging. This alloy is, however, expensive because much zirconium is contained as alloy element, and is poor in corrosion resistance.
On the other hand, as a corrosion resistant favorable forging material, materials undergoing plastic processing such as extrusion after casting of AZ80 alloy are presented, but they are also expensive and are not always suited as alloys for strength member such as wheels. Yet, members such as wheels require extrusion materials for large-sized forged parts, but it is difficult in reality.
To be used as wheels or the like, therefore, large-sized forging part materials must be presented by continuous casting method, but since impact value is important as well as tensile strength and elongation, a greater wall thickness is needed in order to achieve the same strength as that of aluminum forging material, which is contrary to the weight reducing effect. Application of magnesium alloy to large strength member must utilize a continuous casting material having a micro crystal grain. Methods are hence proposed to refine the mean crystal grain size of magnesium alloy cast material (casting billet). One of the methods is disclosed in the Japanese Patent Tokkaisho No. 63-282232, in which molten magnesium alloy is continuously cast at a solidification rate of 25.degree. C./sec or more, or magnesium alloy ingot is plastically processed at 220 to 450.degree. C. at a processing rate of 25% or more. Other proposed methods include a casting method of refining the mean crystal grain size by adding an refining agent.